A large number of electronic circuits today make use of voltage-controlled oscillators (VCOs). One particularly important field of application is mobile communication, in which a voltage-controlled oscillator is used within a phase-locked loop circuit in order to produce an output frequency with high stability and accuracy.
The most important chip technology for system-on-chip (SOC) applications today is CMOS technology. The continual reduction in the critical dimensions of this technology is accompanied by a reduction in the supply voltages too. Analogue circuit technologies are not compatible with this scaling performed for digital circuits, however. If the supply voltage is scaled using the circuit dimensions, the greatest impairment in a VCO circuit is a reduced tuning range. This means that only a smaller frequency range can be covered, which results in a problem for mass production on account of technological tolerances and temperature variations.
The VCO concepts known in the prior art typically use varactor diodes with a large tuning range in order to be able to cover the desired frequency range. FIG. 1 is a schematic illustration of a voltage-controlled oscillator circuit 10 based on the prior art. This LC resonant circuit contains coil elements 1.1 and 1.2 arranged in one circuit path and varactors 2.1 and 2.2 arranged in a circuit path connected in parallel therewith. The varactors 2.1 and 2.2 are voltage-dependent capacitance elements in which the capacitance can be adjusted variably within a particular prescribed range by an analogue voltage signal Vtune which is supplied to the respective source inputs of said capacitance elements. The resonator is thus formed from the inductive coil elements 1.1 and 1.2 and the varactors 2.1 and 2.2. The two transistors 3.1 and 3.2 with the cross-coupled gates ensure the necessary ring gain through positive feedback. The output voltage signal from the VCO oscillator can be taken at the outputs A and B.
The capacitance range of the varactors 2.1 and 2.2, which determines the operating frequency range of the VCO circuit, needs to be designed such that allowance is also made for effects of temperature and process tolerances from the outset. However, the conventional VCO concept described has particular drawbacks on account of the fact that in submicron CMOS technology with critical dimensions of 0.13 μm, for example, the supply voltage is scaled, i.e. reduced, accordingly. One drawback is that the tuning range in this VCO circuitry is likewise reduced on account of the lower supply voltage. Since the capacitance ratio Cmax/Cmin of the varactor diodes remains almost constant, the tuning range of the output frequency is reduced, since the supply voltage is reduced. It is therefore necessary to take suitable measures to ensure the full performance of the VCO circuit over the entire desired frequency range while taking into account production tolerances and temperature variations.